Halogen‐Free π‐Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility

Abstract In π‐conjugated polymers, a highly ordered backbone structure and solubility are always in a trade‐off relationship that must be overcome to realize highly efficient and solution‐processable organic photovoltaics (OPVs). Here, it is shown that a π‐conjugated polymer based on a novel thiazole‐fused ring, thieno[2′,3′:5,6]benzo[1,2‐d:4,3‐d′]bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent‐shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron‐deficient nature of these thiazole‐fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open‐circuit voltages (V OCs) in OPV cells, even without halogen substituents that are commonly introduced into high‐performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and V OCs of more than 0.84 V are observed, both of which are among the top values reported so far for “halogen‐free” polymers. This study will serve as an important reference for designing π‐conjugated polymers to achieve highly efficient and solution‐processable OPVs.

Bis(trimethytin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b:4,5-b′]dithiophene was purchased from Ossila Ltd. Polymerization was carried out using a microwave reactor, Biotage Initiator. Molecular weights of the polymers were evaluated by a high-temperature GPC For the solubility test of the polymers, 2 mg of a polymer sample was prepared in a vial. 0.1 mL of chloroform (CF) was first added and stirred at 50 °C, and then CF was further added until the polymer sample was completely dissolved.
1-Formylpiperidine (2.13 mL, 19.2 mmol) was then added at −78 °C, and the solution was further stirred for 3 h, during which time the solution was warmed to room temperature. The reaction was quenched with water, and the resulting mixture was extracted with hexane. The organic layer was washed with water three times and was dried over anhydrous sodium sulfate.
After the solvent was evaporated under a reduced pressure, the crude product was purified by column chromatography on silica gel with dichloromethane to give 2 as yellow oil (2.70 g, 84%  185.10, 173.63, 152.50, 135.15, 31.58, 18.44, 11.60.

2,7-Bis(triisopropylsilyl)benzo[1,2-d:4,3-d′]bis(thiazole) (3)
To a mixture of zinc (3.90 g, 60.3 mmol) and THF (200 mL), titanium(IV) chloride (3.30 mL, 30.0 mmol) was added dropwise at 0 °C. The reaction mixture was then stirred at 80 °C for 3 h. 2 (2.70 g, 5.00 mmol) in THF (100 mL) was added dropwise, and the reaction mixture was stirred at 80 °C for 24 h. After cooling to room temperature, aqueous sodium sulfate was added and the resulting mixture was extracted with hexane. The organic layer was washed with water three times and was dried over anhydrous sodium sulfate. After the solvent was evaporated under a reduced pressure, the crude product was purified by column chromatography on silica gel with dichloromethane to give 3 as yellow solid (1.77 g, 70% δ: 172.13, 154.09, 128.98, 121.20, 18.50, 11.76.

Benzo[1,2-d:4,3-d′]bisthiazole (4)
To a solution of 3 (1.77 g, 3.50 mmol) in THF (30 mL), tetrabutylammonium fluoride (10.5 mL, 10.5 mmol) was added dropwise at 0 °C. The reaction solution was then stirred at 0 °C for 1 h. The reaction was quenched with water, and then extracted with ethyl acetate. The organic layer was washed with water and dried over anhydrous sodium sulfate. After the solvent was evaporated under a reduced pressure, the residue was purified by column chromatography on silica gel from chloroform (CF):ethyl acetate (7:1) to give 4 as white solid (505 mg, 75%).

PTBTz2
To Tapping mode atomic force microscopy was carried out on a SPM-9700HT scanning probe microscope (Shimadzu Corp). Transmission electron microscopy (TEM) was conducted on JEM-2021 (JEOL).

Grazing incidence wide-angle X-ray diffraction (GIXD) measurements
GIXD measurements were performed with a HUBER multi-axis diffractometer installed in the beamline BL46XU at SPring-8. The X-ray beam from the undulator was monochromatized by a double-crystal Si(111) monochromator. The X-ray energy was 12.39 keV (λ = 1 Å), and the X-ray beam size was 40 µm (height) × 300 µm (width) at the sample position. The diffraction from the samples was detected by a two-dimensional (2D) X-ray photon counting pixel detector (PILATUS 300 K). The X-ray beam incidence angle was set to 0.12 °, and the camera length (sample-to-detector distance) was set to 174 mm. The neat films were prepared by spin-coating the material solution in chloroform on the PEDOT:PSS coated ITO glass with S32 the same conditions, where the substrate size was 1 cm × 1 cm. For the blend films, the photoactive layer on the OPV cells were directly used for the measurements. The measurements were performed in air at room temperature. The exposure time was 1 s, and no irradiation damage was observed on the samples. The coherence length (L) was estimated from the simplified Scherrer's equation, L = 2π/fwhm, [S8] where fwhm is the full-width at half-maximum of the lamellar and π-π stacking diffraction peaks. Note that fwhm was not corrected for the resolution function typically caused by the sample size.

Time-resolved microwave conductivity (TRMC) measurement
A resonant cavity was used to obtain a high degree of sensitivity in the conductivity measurement. The resonant frequency and microwave power were set at ca. 9.1 GHz and 3 mW, respectively, so that the electric field of the microwave was sufficiently small to not disturb the motion of charge carriers. The third harmonic generation (THG; 355nm) of a Nd:YAG laser (Continuum Inc., Surelite II, 5-8 ns pulse duration, 10 Hz) was used as an excitation source.
The incident photon intensity was 9.1 × 10 15 photons cm −2 pulse −1 . The photoconductivity transient Δσ is converted to the product of the quantum efficiency φ and the sum of charge carrier mobilities Σμ, by φΣμ =Δσ(eI0Flight) −1 , where e and Flight are the unit charge of a single electron and a correction (or filling) factor.

Fabrication and measurement of photovoltaic cells
ITO substrates were first pre-cleaned sequentially by sonicating in a detergent bath, deionized water, acetone, and isopropanol at room temperature, and in boiled isopropanol each for 10 min, and then baked at 120 °C for 10 min in air. The substrates were subjected to UV/ozone treatment at room temperature for 10 min.

Fabrication and measurement of hole-and electron-only devices
For hole-only devices, the pre-cleaned ITO substrates were coated with PEDOT:PSS by spin-coating (5000 rpm for 30 s, thickness of ~30 nm) in air. The polymer/Y6 (1:1.2 w/w) blend film was then fabricated by spin-coating in the glove box as described for the photovoltaic cells. MoOx (7.5 nm) and Ag (200 nm) layers were deposited sequentially through a shadow mask in the vacuum evaporator. For electron-only devices, the pre-cleaned ITO substrates were coated with ZnO nanoparticle suspension (1200 rpm for 20 s) in air, and then the polymer/Y6 (1:1.2 w/w) blend film in the glove box as mentioned above. The ZnO layer was again deposited by spin-coating (6000 rpm for 20 s) in air, and then the Ag layer (200 nm) was deposited by vacuum evaporation.
The J-V characteristics were measured in the range of 0-7 V using a Keithley 2400 sourcemeasure unit in the dark in the glovebox. The J-V characteristics were measured at room temperature. The mobility (µ) was calculated by fitting the J-V curves according to the space charge limited current model described by the following equation S3: where εr is the relative dielectric constant of the polymer, ε0 is the permittivity of free space, V = Vappl − Vbi, where Vappl is the applied voltage to the device and Vbi is the built-in voltage due S34 to the difference in work function of the two electrodes, which was determined to be 0.1 V, and L is the polymer thickness. The dielectric constant εr is assumed to be 3, which is a typical value for semiconducting polymers.

Contact angle measurement and surface energy calculation.
Droplets of ultrapure water and glycerol were dripped onto the thin film polymers and Y6.
The γ d and γ p can be calculated through the formula below based on the contact angles obtained by two different solvents, where θ is the contact angle of a solvent, γL is the surface energy of the solvent, γS d and γL d refer to the dispersive surface energy of the material and the specific solvent, respectively, and γS p and γL p refer to the polar surface energy of the material and the specific solvent, respectively.
Thus, the unknown value γS d and γS p can be solved by combining two equations obtained by contact angle measurement of two different solvents.
S35 Figure S1. GPC charts of the polymers.    a) The coherence length (L) for the π-π stacking order, which was estimated from the simplified Scherrer's equation, L = 2π/fwhm, where fwhm is the full-width at half-maximum.
S39 Figure S8. UV-Vis absorption spectra of (a) the polymer solutions, (b) polymer neat films, and (c) polymer/Y6 blend films with the absorption coefficient.  a) The coherence length (L) for the π-π stacking order, which was estimated from the simplified Scherrer's equation, L = 2π/fwhm, where fwhm is the full-width at half-maximum.   Figure S11. (a) J-V curves and (b) EQE spectra of PiNBTz1/Y6 cells with different p/n ratios.      Figure S14. (a) J-V curves and (b) EQE spectra of PTBTz2/Y6 cells with different p/n ratios.    Figure S16.         Figure S20. EQE spectra of the (a) PNBTz1/Y6, (b) PTBTz2/Y6 cells for the polymers with different molecular weights.